Hydrophilized porous membrane and production process thereof

ABSTRACT

The present invention discloses a hydrophilized porous membrane with a crosslinked hydrophilic polymer, which is composed principally of diacetone acrylamide, held physically on at least a part of the pore walls of a starting porous polyolefin membrane, as well as its production process. This hydrophilized porous polyolefin membrane has long-lasting hydrophilicity and good mechanical strength. Its components are dissolved out only minimally in application.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to hydrophilized porous membranes useful in suchfields as water treatment and blood purification and to their productionprocess, and more specifically to porous polyolefin membranes with theirpores covered by a hydrophilic polymer and to a production processthereof.

(2) Description of the Prior Art

The fields of application of porous polyolefin membranes are expandingrapidly due to their excellent mechanical properties and chemicalresistance. Porous polyolefin membranes are however hydrophobic and whenused as is, water is allowed to permeate therethrough with difficulty. Ahydrophilizing treatment is therefore indispensable to have hydrophilicliquids including water permeate therethrough. A variety of methods havebeen studied with a view toward imparting hydrophilicity through surfacemodification of polyolefin membranes. Hydrophilizing methods, which havebeen proposed for film-like materials featuring smooth surfaces, cannotbe simply applied to impart hydrophilicity to porous membranes havingcomplex surface configurations.

As hydrophilizing methods for porous polyolefin membranes, there havebeen known the organic solvent wetting and water substituting method inwhich the entire surface of a porous polyolefin membrane, inclusive ofminute pores, is subjected to a wetting treatment with an organicsolvent having good miscibility with water such as an alcohol or ketone,followed by substitution of water for the organic solvent; the physicaladsorption method in which a hydrophilic material such as polyethyleneglycol or a surfactant is adsorbed on a surface of a porous membrane soas to impart hydrophilicity to the porous membrane (Japanese PatentLaid-Open Nos. 153872/1979 and 24732/1984); and the chemical surfacemodification method in which a porous membrane is exposed to radiationwhile holding a hydrophilic monomer on a surface of the membrane(Japanese Patent Laid-Open No. 38333/1981) or the porous structure of ahydrophobic resin is subjected to a plasma treatment in a stateimpregnated with a water-soluble high-molecular material and asurfactant (Japanese Patent Laid-Open No. 157437/1981).

In the organic solvent wetting and water substituting method, if wateris once lost from minute pores during storage or use, the partcontaining these water-free minute pores regains hydrophobicity and nolonger permits the permeation of water therethrough. Accordingly, sinceit is always necessary to keep water around the porous membrane, theporous membrane is difficult to handle. Although the physical adsorptionmethod is easy to practice, the hydrophilic material drops off if theresulting porous membrane is used over a long period of time. Therefore,this method cannot be regarded as a fully satisfactory hydrophilizingmethod. The conventional chemical surface modification method isaccompanied by one or more problems. It is difficult to impart uniformhydrophilicity in the direction of the thickness of a membrane, when theporous membrane is exposed to radiation or subjected to the plasmatreatment. If one attempts to apply a hydrophilizing treatment uniformlyover the entire thickness of a porous membrane when the membrane has alarge thickness or is in the form of a hollow fiber, the mechanicalstrength of the matrix of the porous membrane is unavoidably reduced,leading to damage.

SUMMARY OF THE INVENTION

An object of this invention is to provide a porous polyolefin membraneholding firmly a hydrophilic polymer over almost all of the pore wallsof the membrane, imparted with hydrophilicity of excellent durabilityand having sufficient mechanical strength.

In one aspect of this invention, there is thus provided a hydrophilizedporous membrane, wherein a crosslinked hydrophilic polymer composedprincipally of diacetone acrylamide is held on at least a part of thepore walls of a starting porous polyolefin membrane.

In another aspect of this invention, there is also provided a processfor the production of the aforementioned hydrophilized porous membrane,which comprises the steps of (A) holding diacetone acrylamide, acrosslinkable monomer and a polymerization initiator on at least a partof the pore walls of a starting porous polyolefin membrane, and (B)heating them to polymerize these monomers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, the polyolefin forming the porous polyolefinmembrane includes a polymer or copolymer composed principally (80 wt.%or more) of one or more monomers selected from the group consisting ofethylene, propylene, 4-methyl-1-1-pentene and 3-methyl-1-butene, and thefluorinated (co)polymer thereof. The starting porous membrane may be inany form such as a hollow fiber membrane, planar membrane or tubularmembrane. Although starting porous membranes having various pore sizesmay be employed depending on the end use, the preferred starting porousmembranes may include those having a membrane thickness of about 20-200μm, a porosity of about 20-90 vol.%, a water permeability of about0.001-10 l/min·hr·mmHg as measured by the alcohol-dependenthydrophilizing method, and a pore size of about 0.01-5 μm.

There are various pore structures of porous polyolefin membranes. Ofthese, porous polyolefin membranes obtained by the stretching method canbe used preferably from the viewpoint that their porosity is high andthey are hence less susceptible to performance drop due to clotting.Porous membranes obtained by the stretching method are those having aslit-like pore structure in which minute spacings (pores) formed bymicrofibriles and knot portions are communicated mutually in athree-dimensional pattern. They may be produced, for example, by theprocess disclosed in U.S. Pat. No. 4,055,696 or 4,401,567.

As to the shapes of the porous membranes, hollow fiber membranes arepreferably used because they have large membrane areas per unit volume.

By the term "at least a part of the pore walls" of the porous polyolefinmembrane of this invention, on which a crosslinked hydrophilic polymeris held, is meant a part or the entire part of the pore walls.

It is sufficient if the crosslinked hydrophilic polymer is held on thepore walls to the extent that an acceptable flow rate is achievedthrough the membrane when the porous membrane is used by allowing waterto permeate through its pores under the usual intermembrane pressuredifference. It is not absolutely necessary to cover the entire porewalls with the polymer. Furthermore, the hydrophilic polymer may be heldor may not be held on the outer surfaces of the porous membrane.

By the term "physically held" as used herein means that the polymer isbonded or otherwise adhered firmly to the pore walls to such a degreethat the polymer does not drop off easily in the course of storage oruse of the porous membrane. The polymer may firmly adhere to the porewalls by anchorage effects. Alternatively, the polymer may be adherentlycrosslinked in such a manner that it encloses the microfibriles or knotportions, which form the slit-like pores.

A hydrophilic polymer may also be held on the pore walls of a porouspolyolefin membrane primarily by chemical bonds. This type of bonding ishowever not preferable, because the matrix of the membrane, such asmicrofibriles, is damaged upon bonding the polymer thereon, resulting ina modification to the pore structure of the porous membrane or areduction to its mechanical strength. In the hydrophilized porousmembrane of this invention, some chemical bonds may however existbetween the porous polyolefin membrane and the crosslinked hydrophilicpolymer so long as the existence of the chemical bonds does not raiseany practical problem.

In the present invention, a crosslinked hydrophilic polymer composedprincipally of diacetone acrylamide is held on the pore walls of aporous polyolefin membrane. This polymer has been selected for thefollowing reasons. Compared with other polymers, (1) the above polymercan adhere firmly to a polyolefin and can hence be held firmly there;(2) it can be held almost uniformly over substantially the entire porewalls of a porous polyolefin membrane; (3) it has a suitable degree ofhydrophilicity; and (4) it is substantially water-insoluble.

The term "crosslinked hydrophilic polymer composed principally ofdiacetone acrylamide" as used herein means a polymer containing 50 wt.%or more of diacetone acrylamide [N-(1,1-dimethyl-3-oxobutyl)acrylamide]as a monomer component. As a copolymerizable monomer also composing thepolymer, a crosslinkable monomer is used. However, a non-crosslinkablemonomer may also be used in combination.

Such a copolymerizable monomer is a monomer which is copolymerizablewith diacetone acrylamide and contains at least one polymerizableunsaturated bond such as vinyl bond or allyl bond, and has a goodsolvent common to diacetone acrylamide.

As an exemplary crosslinkable monomer, may be mentioned a monomercontaining at least two polymerizable unsaturated bonds such as thosementioned above, or a monomer containing one of such polymerizableunsaturated bonds as those mentioned above and at least one functionalgroup capable of forming a chemical bond by, for example, a condensationreaction. Illustrative examples of such a crosslinkable monomer mayinclude N,N'-methylenebisacrylamide, N-hydroxymethylacrylamide,N-hydroxymethylmethacrylamide, triallyl cyanurate, triallylisocyanurate, divinyl benzene, 2,2-bis(4-methacryloyloxypolyethoxyphenyl)propane, ethylene di(meth)acrylate,polyethyleneglycol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate,trimethylolethane tri(meth)acrylate, butanediol di(meth)acrylate,hexanediol di(meth)acrylate, diallyl phthalate and 1,3,5-triacryloylhexahydroxy-s-triazine.

On the other hand, as exemplary non-crosslinkable monomers, may bementioned dimethylmethacrylamide, vinylpyrrolidone, acrylic acid,methacrylic acid, hydroxyethyl methacrylate, styrenesulfonic acid,sodium styrenesulfonate, sodium sulfoethylmethacrylate, vinylpyridineand vinyl methyl ether.

Regarding the proportions of diacetone acrylamide and copolymerizablemonomer which in combination form the crosslinked hydrophilic copolymer,it is preferable to use the copolymerizable monomer in an amount ofabout 0.5-100 parts by weight per 100 parts by weight of diacetoneacrylamide.

Since the hydrophilic polymer held on the pore walls of the porouspolyolefin membrane is a crosslinked polymer in this invention, thehydrophilic polymer held on the pore walls of the porous polyolefinmembrane undergoes only a small degree of swelling in water and hasalmost no potential danger to plug the pores. The hydrophilic polymerhas further advantages that its stability is good and its components aredissolved out very little in water. The porous membrane is thereforeeffective in the field of water treatment or blood purification, wheredissolved components cause problems even at trace levels.

By contrast, a diacetone acrylamide polymer having no crosslinkedstructure undergoes swelling in water, reduces pore size and sometimesplugs pores. It also dissolves in water, albeit in a small amount. Aporous membrane with such a hydrophilic polymer held thereon has apotential danger of developing various problems upon its application.

The greater the degree of hydrophilicity of the crosslinked polymer, thebetter the performance of water permeation by the resulting porousmembrane. A water-soluble and crosslinkable monomer having a sufficientdegree of hydrophilicity is preferable as the crosslinkable monomer forthe formation of the crosslinked polymer, since water is allowed topermeate evenly through the entire membrane area of the resulting porousmembrane in a short period of time after starting its use.

Such a water-soluble and crosslinkable monomer is a crosslinkablemonomer having a solubility of 1.0 g/dl or greater in water of 30° C. Asillustrative examples of the crosslinkable monomer, may be mentionedN-hydroxymethylacrylamide, N-hydroxymethylmethacrylamide andN,N'-methylenebisacrylamide.

The amount of the crosslinked hydrophilic polymer held on at least apart of pore walls of a porous polyolefin membrane according to thisinvention is dependent on the porosity and pore size of the porouspolyolefin membrane but is preferably about 0.5-100 wt. % based on theweight of the porous polyolefin membrane. If the amount of the thus-heldcrosslinked polymer is smaller than the lower limit, it is impossible toimpart sufficient hydrophilicity to the porous membrane. On the otherhand, any amounts greater than the upper limit cannot improve thehydrophilicity of the porous membrane any further. On the contrary, thevolume of each pore is reduced so that the performance of waterpermeation is lowered. The amount of the thus-held polymer is morepreferably about 0.5-50wt. %, and most preferably about 1-30 wt. %.

A description will next be made of processes for the production of thehydrophilized porous membranes of this invention.

A variety of processes may be employed to hold the crosslinkedhydrophilic polymer on the pore walls of the porous polyolefin membraneof this invention. The following method may be employed by way ofexample: A solution of diacetone acrylamide and the aforementionedcopolymerizable monomer (hereinafter called "monomers" collectively) anda polymerization catalyst dissolved in a suitable solvent such as anorganic solvent or water is prepared. A starting porous polyolefinmembrane is then impregnated by the above solution by immersing thestarting porous polyolefin membrane in the solution or by fabricating amembrane module with the starting porous polyolefin membrane and thencausing the solution to penetrate under pressure into the porouspolyolefin membrane, followed by evaporation of the solvent for removal.It is possible to cause the monomers to adhere almost uniformly over theentire surface of the porous membrane without plugging the pores of theporous membrane by using the monomers in a form diluted with a solvent.The amounts of the monomers to be adhered can be adjusted by changingthe concentrations of the monomers in the solution or changing theimmersion or penetration time.

After holding these monomers on at least a part of the pore walls of theporous membrane in the above-described manner, the solvent is removedand the monomers are then polymerized. It is hence possible to hold theresulting crosslinked hydrophilic polymer on at least said part of thepore walls of the porous polyolefin membrane.

The solvent useful upon preparation of the above-described solutionincludes water or an organic solvent which has a boiling point lowerthan the monomers and can dissolve the monomers therein. When apolymerization catalyst is added, it is desirable to use a solvent whichcan also dissolve the polymerization catalyst.

As such organic solvents, may be mentioned alcohols such as methanol,ethanol, propanol and isopropanol, ketones such as acetone, methyl ethylketone and methyl isobutyl ketone, ethers such as tetrahydrofuran anddioxane and ethyl acetate. Although no particular limitation is imposedon the organic solvent, the organic solvent preferably has a boilingpoint below about 100° C. and more preferably below about 80° C. becausesuch a boiling point facilitates the removal of the solvent before thepolymerization step.

Since the surface of a porous polyolefin membrane is hydrophobic, themonomers tend to be adsorbed on the pore walls with their hydrophilicgroups oriented outward upon penetration of an aqueous solutioncontaining the monomers into the pores, especially when water is used asa solvent. If the monomers are fixed in this state by polymerization,hydrophilicity can be imparted with extremely high efficiency. Whenwater is used as a solvent, it is possible to bring the starting porousmembrane into contact with the resulting solution without anypretreatment. It is also possible to bring the starting porous membraneinto contact with the solution after subjecting the pore walls of theporous membrane to a wetting treatment with an alcohol or ketone inadvance.

By contrast, use of an organic solvent as the solvent has the merit thatthe resulting solution is allowed to penetrate into pores of a porouspolyolefin membrane in a short period of time and that the solvent canbe removed with ease from the pores.

Even when the monomers are polymerized in a state oriented at random onthe pore walls instead of making use of the above-mentioned orientedadsorption, the resulting hydrophilic polymer has a greater degree ofhydrophilicity compared to polyolefins. Compared with pore walls notholding the crosslinked polymer thereon, pore walls with the crosslinkedpolymer held thereon have higher hydrophilicity. It is hence possible toobtain the porous polyolefin membrane in a form imparted withhydrophilicity.

Need for a polymerization initiator is dependent on the manner ofpolymerization. A polymerization initiator is employed in heatpolymerization or photopolymerization, but no polymerization initiatoris required for radiation polymerization.

In the case of heat polymerization, it is possible to use variousperoxides, azo compounds and redox initiators which are known as radicalpolymerization initiators. As their examples, may be mentioned azocompounds such as 2,2'-azobisisobutylonitrile,2,2'-azobiscyclopropylpropionitrile,2,2'-azobis-2,4-dimethylvaleronitrile, and2,2'-azobis-2,3,3-trimethylbutylonitrile; peroxides such as acetylperoxide, propionyl peroxide, butyryl peroxide, isobutyryl peroxide,succinyl peroxide, benzoyl peroxide, benzoylisobutyryl peroxide,β-allyloxypropionyl peroxide, hexanoyl peroxide, 3-bromobenzoyl peroxideand bis-(4-t-butylcyclohexyl) peroxydicarbonate; and persulfates such aspotassium persulfate and ammonium persulfate.

A water-soluble polymerization initiator such as azobisisobutyramidineor 4,4'-azobis-4-cyanopentanoic acid is preferable especially when wateris used as a solvent. However, the above-mentioned water-insolublepolymerization initiators may also be employed because they can still bedispersed in water owing to the surface activity of the monomersthemselves.

In the case of photopolymerization, it is possible to usephotopolymerization catalysts, for example, benzophenone, benzoinmethylether, benzyl dimethyl ketal, fluorenone, 4-bromobenzophenone,4-chloro-benzophenone, methyl 2-benzoylbenzoate, benzoyl peroxide,anthraquinone, biacetyl and uranyl nitrate. They may also be used insuitable combination.

The proportions of the monomers and the solvent in a solution may besuitably chosen in view of the type of the solvent, the target amount ofthe resulting crosslinked hydrophilic polymer to be held and otherfactors. Per 100 parts by weight of the monomers, the solvent may beused in an amount of about 50-10,000 parts by weight, and morepreferably, about 200-5,000 parts by weight.

The proportions of diacetone acrylamide and the copolymerizable monomerin the monomer mixture may be suitable selected in view of the degree ofhydrophilicity of the copolymerizable monomer, target copolymerizationratio and crosslinking degree and other factors. Per 100 parts by weightof diacetone acrylamide, the copolymerizable monomer may be used in anamount of about 0.5-100 parts by weight, and more preferably, about 1-90parts by weight.

Further, per 100 parts by weight of the monomers, the polymerizationinitiator may be used in an amount of about 0.001-100 parts by weightwith about, 0.01-30 parts by weight being more preferred and about0.1-20 parts by weight being particularly preferred.

If the solvent is used in any amount greater than the upper limit of theabove range relative to the monomers, the amounts of the monomers to beheld on the pore walls of the porous membrane will be too little to holdthe resulting crosslinked hydrophilic polymer in a sufficient amount. Ifthe amount of the solvent is smaller than the lower limit, difficultieswill be encountered in controlling the amount of the resulting polymerto be held. In addition, the crosslinked polymer will be held too muchon the pore walls and within the pores, leading to plugging of thepores. It is hence not desirable to use the solvent in any amountsoutside the above range.

When a starting porous polyolefin membrane is subjected to an immersionor penetration treatment by using the above-described solution, theimmersion or penetration time may be about 0.5 second-30 minutes. Thistreatment can be effected in a shorter period of time as the wettingcharacteristics of the solution for the porous polyolefin membranebecome better.

After holding the monomers and, in some instances, the polymerizationinitiator on at least a part of the pore walls of the porous polyolefinmembrane in the above-described manner, the accompanying extra solutionis removed and if necessary the solvent penetrated in pores is caused toevaporate, followed by a polymerization step.

If the temperature becomes unduly high during the evaporation andremoval of the solvent, the polymerization is caused to proceed partlywhile the solvent still remains. The polymerization thus takes place inthe interior of pores instead of the pore walls of the porous membraneand as a result, some pores may be plugged. Use of such a hightemperature is therefore not desirable. In view of this possibleproblem, it is preferable to control the temperature within a range of10-40° C. during the removal of the solvent.

In the present invention, polymerization processes such as heatpolymerization, photopolymerization, radiation polymerization and plasmapolymerization may be used.

In heat polymerization, the polymerization temperature is above thedecomposition temperature of the above-mentioned polymerizationcatalyst. It is also desirable not to exceed a temperature at which themembrane structure of the porous polyolefin membrane is changed and thematrix of the membrane is damaged. It is generally preferable to use atemperature of 30-100° C. Although the heating time depends on the typeof polymerization catalyst and the heating temperature, it is generallyabout 1 minute-5 hours and more preferably about 15 minutes-3 hours in abatch process. Since the heat transfer efficiency is higher in acontinuous process, the polymerization can be achieved in a shorterperiod of time. Therefore, the heating time may usually be about 10seconds-60 minutes, with about 20 seconds-10 minutes being preferred.

In photopolymerization, ultraviolet rays or visible light can be used asthe light to be irradiated. As the ultraviolet ray source, alow-pressure mercury lamp, high-pressure mercury lamp, xenon lamp, arclamp or similar lamp may be used.

It is necessary to choose suitable conditions for the irradiation of thelight. When a mercury vapor lamp is used as an exemplary light source,it is necessary to set the input at about 10-300 W/cm and to irradiatelight for about 0.5-300 seconds at a distance of about 10-50 cm so thatthe porous polyolefin membrane is exposed to light with energy of about0.001-10 joule/cm², or more preferably about 0.05-1 joule/cm².

If the intensity of the irradiated light is too small, it is difficultto achieve sufficient hydrophilization. On the other hand, a highirradiation intensity causes considerable damage to the porouspolyolefin membrane. It is hence desirable to choose suitable lightirradiation conditions with care while taking the membrane thickness andother factors into consideration.

In the case of radiation polymerization, the polymerization can beconducted, for example, by irradiating electron beams to about 10-50Mrad at a temperature below 120° C., more preferably below 100° C., bymeans of an electron beam irradiation apparatus.

If oxygen exists in the atmosphere upon polymerization, thepolymerization reaction is significantly impaired. It is thereforedesirable to effect the polymerization in a substantially oxygen-freestate, for example, in an inert gas atmosphere such as a nitrogen gasatmosphere or in vacuo.

When the crosslinked hydrophilic polymer is formed by using acrosslinkable monomer, the crosslinking reaction may be allowed toproceed concurrently with the polymerization reaction. Alternatively, itmay be effected subsequent to the formation of a copolymer. Thecrosslinking reaction, where it depends on condensation, may be effectedby using the heat of the polymerization reaction or by heating thepolymerization system externally.

When a condensation-dependent crosslinking reaction is used, thecrosslinking reaction may be conducted by dissolving an uncrosslinkedcopolymer of diacetone acrylamide and a crosslinkable monomer, whichcopolymer has been prepared beforehand, in a solvent, holding theresultant solution on the pore walls of the porous polyolefin membraneand then subjecting the copolymer to the crosslinking reaction in thatstate. Here, the uncrosslinked copolymer preferably has a molecularweight of about 10,000-500,000. If its molecular weight is unduly large,it is difficult to have the copolymer penetrate into the pores of theporous polyolefin membrane. Use of such a large molecular weight istherefore not desirable. The more preferable molecular weight is about50,000-300,000.

As described above, various polymerization processes can be employed inthe present invention. It is however most preferable to effect thepolymerization by thermal energy. Since use of thermal energy allowseven pore portions of the porous membrane to be heated evenly, themonomers can be uniformly polymerized over the entire pore walls onwhich they are held. Heat polymerization has another advantage in thatthe polymerization can be achieved without modification of the membranestructure and deterioration of the membrane matrix if the polymerizationtemperature is suitably chosen. By contrast, the use of light energyinvolves the problem that the light cannot sufficiently reach the poreportions of the porous membrane due to scattering of the light. If theirradiation intensity of the light is increased, a further problem isdeveloped in that the deterioration of the matrix of the membrane isaccelerated. Furthermore, the use of radiation energy is accompanied bythe drawback that the membrane matrix is liable to accelerateddeterioration. When these polymerization processes are employed, it ishence indispensable to choose with care polymerization conditions thatdo not cause deterioration of the membrane matrix.

Since the monomers or uncrosslinked copolymer held on the pore walls ofthe porous polyolefin membrane are polymerized or crosslinked in situ byany of the above-described polymerization processes, at least a part ofthe pore walls of the porous membrane is covered by the resultantpolymer.

It is also desirable to remove unnecessary materials such as unreactedmonomers or free polymer with an appropriate solvent subsequent to theformation of the crosslinked hydrophilic polymer. As the solvent, water,organic solvents or their mixed solvents can be used either singly or incombination.

The hydrophilized porous membrane of this invention can be produced inthe above-described manner. As a particularly preferable process, may bementioned heating and polymerizing monomers, which include diacetoneacrylamide and a water-soluble crosslinkable monomer, and in someinstances, a polymerization initiator on at least a part of the porewalls of a porous polyolefin membrane so that they are held there.

The use of a water-soluble crosslinkable monomer as the copolymerizablemonomer can suppress the swelling of the resulting hydrophilic polymerin water, whereby the amounts of components to be dissolved out can bereduced further and at the same time, the hydrophilized porous membraneexhibits excellent water permeation performance. A hydrophilized porousmembrane produced by heat polymerization has the merit that thecrosslinked polymer is held uniformly in the direction of the thicknessof the membrane and the matrix of the membrane is substantially free ofdamage.

The individual steps of the process of this invention have beenseparately described above. It should however be noted that suchindividual steps as holding of the monomers on the pore walls of aporous polyolefin membrane, removal of the solvent, polymerization andwashing after the polymerization can be performed continuously in thepresent invention.

According to the present invention, it is possible to hold a crosslinkedhydrophilic polymer firmly on the pore walls in a porous polyolefinmembrane without lowering the mechanical strength of the matrix of theporous polyolefin membrane.

Compared with a porous polyolefin membrane holding no hydrophilicpolymer thereon, the hydrophilized porous membrane of this inventionrequires a significantly low water penetration pressure and thereforehas extremely good water permeation performance. Since the crosslinkedpolymer is held firmly on the pore walls of the porous polyolefinmembrane, its components are dissolved out very little even in adissolving-out test in warm water. The hydrophilized porous membrane ofthis invention can therefore be used successfully in such fields aswater treatment and blood purification, where high-temperaturetreatments may be involved.

In particular, a hydrophilized porous membrane obtained from apolyolefin membrane rendered porous by the stretching technique has themerit that it exhibits good hydrophilicity and that the increase infiltration resistance due to plugging upon application of the membraneis minimized.

The present invention will hereinafter be described specifically by thefollowing Examples. In each Example, a porous polyolefin membranerendered porous by the stretching technique in which slit-like spacings(pores) formed by fibriles and knots communicated three-dimensionallywas used and the pore size of the porous membrane was defined in termsof the average width and length of the slit-like spacings. Waterpenetration pressure, water permeability by the alcohol-dependenthydrophilizing method, and water permeability after the holding of apolymer thereon were each measured in accordance with the followingmethods by fabricating test membrane modules each of which had aneffective membrane area of 163 cm². In addition, the amount of polymerheld, the knot strength and the cumulative dissolution (%) were alsomeasured by the following methods, and the evaluation of the state ofcoverage of the pore walls by the polymer was effected by the followingmethod.

The solubilities of N-hydroxy-methylacrylamide, N,N'-methylenebisacrylamide and triallyl isocyanurate which were used in the followingExamples were 197 g/dl, 3 g/dl and 0.1 g/dl respectively.

(1) Water penetration pressure:

Water of 25° C. was fed from one side (the inside of hollow fibers inthe case of a hollow fiber membrane) of a test membrane module whileraising the water pressure at a rate of 0.1 kg/cm² per minute. Waterpressures were separately measured when the cumulative quantity ofpenetrated water had reached 30 ml and 50 ml. The water pressures andquantities of penetrated water were plotted along the axis of abscissaand the axis of ordinate, respectively. The pressure at the crossingpoint between the straight line, which connected the thus-plotted twopoints, and the axis of abscissas was determined. The pressure wasemployed as the water penetration pressure.

(2) Water permeability by the alcohol-dependent hydrophilizing method:

Ethanol was fed under pressure at a flow rate of 25 ml/min for 15minutes from one side (the inside of hollow fibers in the case of ahollow fiber membrane) of a test membrane module which had not beensubjected to any hydrophilizing treatment, whereby the porous membranewas wet fully to the interior of its pores with ethanol. Thereafter,water was caused to flow at a flow rate of 100 ml/min for 15 minutes sothat the ethanol contained within the pores was substituted by water.Then, water of 25° C. was caused to flow from one side (the inside ofhollow fibers in the case of a hollow fiber membrane) of the testmembrane module and the quantity of permeated water was measured at anintermembrane pressure difference of 50 mmHg. The water permeability(l/m² ·hr·mmHg) was determined from the quantity of permeated water.

(3) Amount of crosslinked hydrophilic polymer held:

The nitrogen content was determined by elemental analysis. On theassumption that the nitrogen had been derived only from the crosslinkedhydrophilic polymer, and that the thus-formed crosslinked hydrophilicpolymer had the same composition as the monomer composition, eachpolymer held on a porous polyolefin membrane was measured in terms ofwt.% based on the unit weight of the porous polyolefin membrane.

(4) Evaluation of the state of coverage of pore walls:

Each porous membrane was immersed for 1 minute in the standard solution(blue) for wetting tests described in JIS K6768(1971), which solutionhas a surface tension of 54 dyn/cm. Thereafter, the membrane was driedin air, and a transverse cross-section of the porous membrane wasobserved through an optical microscope to observe the state ofdistribution of the colored crosslinked hydrophilic polymer.

(5) Knot strength:

The knot strength of each porous hollow fiber membrane was measured inaccordance with JIS L1013.

(6) Cumulative dissolution (%):

Each porous membrane was immersed in warm water of 65° C. in an amount10 times the weight of the membrane. At a constant interval, thequantity of the organic carbon atoms in the warm water was analyzed. Onthe assumption that the quantity of the whole organic carbon atoms wasderived only from the crosslinked hydrophilic polymer of the compositionassumed in the above test (3), the cumulative dissolution wascalculated. Then, the cumulative dissolution (wt.%) was determinedrelative to the amount of the crosslinked polymer held before thedissolving-out treatment.

(7) Water permeability after holding the crosslinked hydrophilicpolymer:

After feeding water at a pressure of 2 kg/cm² for 3 hours from one sideof a test membrane module fabricated with a porous membrane holding acrosslinked hydrophilic polymer thereon (the inside of hollow fibers inthe case of a hollow fiber membrane), water of 25° C. was caused to flowfrom the other side of the test membrane module. The quantity ofpermeated water was measured at an intermembrane pressure difference of50 mmHg. The water permeability (l/m² ·hr·mmHg) was determined from thequantity of permeated water.

EXAMPLE 1

A porous polyethylene hollow fiber membrane having a slit-like pore withan average width of 0.4 μm and an average length of 1.8 μm, a porosityof 63%, a membrane thickness of 70 μm, an inner diameter of 270 μm, aknot strength of 394 g/fil, a water penetration pressure of 11 kg/cm²,and a water permeability of 1.1 l/m² ·hr·mmHg as measured by thealcohol-dependent hydrophilizing method, was immersed for 10 seconds ina solution composed of 100 parts of diacetone acrylamide, 5 parts ofN-hydroxymethylacrylamide, 1 part of benzoyl peroxide and 1,000 parts ofacetone. The hollow fiber membrane was thereafter taken out of thesolution and dried in air for 5 minutes. The porous membrane wasthereafter heat-treated at 65° C. for 60 minutes in a nitrogen gasatmosphere and then immersed for 10 minutes in a 50:50 (by parts) mixedsolvent of water and ethanol. By ultrasonic cleaning of the membrane for2 minutes in warm water, unnecessary materials were washed off. Themembrane was then dried in hot air to remove the solvent, therebyobtaining the porous membrane with a crosslinked hydrophilic polymerheld thereon.

The water penetration pressure, water permeability, amount of thecrosslinked polymer held, knot strength and cumulative dissolution (%)of the membrane were measured. The results are shown in Table 1.

The water permeation performance of the resulting hydrophilic porousmembrane was excellent. The crosslinked polymer was held almostuniformly over substantially the entire pore walls of the porousmembrane. In addition, the knot strength did not decrease compared tothe starting porous membrane. It was found that practically no componentwere dissolved out after the 24th hour of dissolving-out test.

EXAMPLES 2-4

Crosslinked hydrophilic polymers were separately held on porousmembranes under the same conditions as in Example 1 except thatN-hydroxymethylacrylamide was used as the crosslinkable monomer in theamounts shown in Table 1.

The performance of the porous membranes obtained in the above manner wasevaluated. The results are also shown in Table 1.

EXAMPLE 5

A porous polyethylene hollow fiber membrane having a slit-like pore withan average width of 0.2 μm and an average length of 0.7 μm, a porosityof 45%, a membrane thickness of 22 μm, an inner diameter of 200 μm, awater penetration pressure of 12 kg/cm² and a water permeability of 0.54l/m² ·hr·mmHg as measured by the alcohol-dependent hydrophilizing methodwas converted into a porous membrane having a crosslinked hydrophilicpolymer held thereon in the same manner as in Example 1 except that asolution composed of 100 parts of diacetone acrylamide, 5 parts ofN,N'-methylene bisacrylamide, 5 parts of 2,2'-azobisisobutyronitrile and800 parts of acetone was used as a treatment solution and the heattreatment was applied at 65° C. for 60 minutes. The performance of theporous membrane was evaluated. The results are shown in Table 1.

The crosslinked hydrophilic polymer was held almost uniformly oversubstantially the entire pore walls. It was also found that practicallyno components were dissolved out after the 24th hour of dissolving-outtest.

EXAMPLES 6-8

Crosslinked hydrophilic polymers were separately held on porousmembranes under the same conditions as in Example 5 except thatN,N'-methylene bisacrylamide was used as the crosslinkable monomer inthe amounts shown in Table 1.

The performance of the porous membranes obtained in the above manner wasevaluated. The results are also shown in Table 1.

EXAMPLE 9

A planar porous polyethylene membrane, which had slit-like pores havingan average width of 0.8 μm and an average length of 3.0 μm, a porosityof 70%, a membrane thickness of 42 μm, a water penetration pressure of4.5 kg/cm² and a water permeability of 3.5 l/m² ·hr·mmHg as measured bythe alcohol-dependent hydrophilizing method, was converted into a porousmembrane having a crosslinked hydrophilic polymer held thereon in thesame manner as in Example 1 except that a solution formed of 100 partsof diacetone acrylamide, 5 parts of N-hydroxymethylacrylamide, 10 partsof benzoyl peroxide and 330 parts of methyl ethyl ketone was used as thetreatment solution and the heat treatment was applied at 60° C. for 60minutes. The performance of the porous membrane was evaluated. Theresults are also given in Table 1.

The crosslinked hydrophilic polymer was held almost uniformly oversubstantially the entire pore walls. It was also found that practicallyno components were dissolved out after the 24th hour of dissolving-outtest.

EXAMPLES 10-12

Crosslinked hydrophilic polymers were separately held on porousmembranes under the same conditions as in Example 9 except thatN-hydroxymethylacrylamide was used as the crosslinkable monomer in theamounts shown in Table 1.

The performance of the porous membranes obtained in the above manner wasevaluated. The results are also shown in Table 1.

EXAMPLE 13

A crosslinked hydrophilic polymer was held on a porous membrane underthe same conditions as in Example 1, except that 5 parts of triallylisocyanurate was used as the crosslinkable monomer.

The performance of the porous membrane obtained in the above manner wasevaluated. The results are also shown in Table 2. The crosslinkedhydrophilic polymer was held almost uniformly over substantially theentire pore walls.

EXAMPLE 14

A crosslinked hydrophilic polymer was held on a porous membrane underthe same conditions as in Example 5, except that 5 parts of triallylisocyanurate was used as the crosslinkable monomer.

The performance of the porous membrane obtained in the above manner wasevaluated. The results are also shown in Table 2.

EXAMPLE 15

A crosslinked hydrophilic polymer was held on a porous membrane underexactly the same conditions as in Example 5, except that a solutioncomposed of 100 parts of diacetone acrylamide, 1 part of divinylbenzene,0.3 part of benzoyl peroxide and 450 parts of methyl ethyl ketone wasused and the immersing time and heat polymerization conditions were setat 3 seconds and at 70° C. for 60 minutes, respectively.

The performance of the above porous membrane was evaluated. The resultsare also shown in Table 2. The crosslinked hydrophilic polymer was heldalmost uniformly over substantially the entire pore walls. It was alsofound from the measurement of the cumulative dissolution (%) thatpractically no components were dissolved out after the 24th hour ofdissolving-out test.

EXAMPLE 16

Using a planar porous polyethylene membrane similar to that employed inExample 9, a hydrophilized porous membrane of this invention wasobtained in the same manner as in Example 9, except that a solutionformed of 100 parts of diacetone acrylamide, 5 parts of triallylisocyanurate, 5 parts of benzoyl peroxide and 300 parts of acetone wasused and the immersing time and heat polymerization conditions were setat 3 seconds and at 60° C. for 30 minutes, respectively.

The performance of the above porous membrane was evaluated. The resultsare also shown in Table 2.

EXAMPLE 17

A hydrophilized porous membrane of this invention was obtained in thesame manner as in Example 1, except that a planar porouspoly-4-methyl-1-pentene membrane containing slit-like pores, the averagewidth and length of which were 0.2 μm and 0.5 μm, respectively, andhaving a porosity of 43%, a membrane thickness of 35 μm and a waterpermeability of 0.2 l/m² ·hr·mmHg as measured by the alcohol-dependenthydrophilizing method was used, the amount of the benzoyl peroxide waschanged to 0.5 part, and the immersion time in the solution and the heatpolymerization conditions were set at 3 seconds and at 75° C. for 25minutes, respectively. The performance of the porous membrane wasevaluated. The results are also shown in Table 2. The crosslinkedhydrophilic polymer was held almost uniformly over substantially theentire pore walls.

EXAMPLES 18-21

Porous polyethylene hollow fiber membrane of the same type as that usedin Example 1 were continuously fed at a speed of 2 m/min through asolution tank of 10 cm long, in which each membrane was subjected to animmersion treatment. In a first pipe having a diameter of 2 cm and alength of 4 m, the accompanying solution was removed and eachthus-immersed membrane was dried. Thereafter, each membrane was heatedin a second pipe having a diameter of 2 cm and a length of 3 m so thatthe monomers were polymerized.

Four types of solutions were employed separately. Their compositionswere as follows:

    ______________________________________                                        Diacetone acrylamide                                                                              100 parts                                                 N--hydroxymethylacrylamide                                                                        See Table 2                                               Bis-(4-tertiary-butylcyclohexyl)                                                                  0.5 part                                                  peroxydicarbonate                                                             Acetone             660 parts                                                 ______________________________________                                    

Nitrogen gas of room temperature and hot nitrogen gas of 80° C. werecaused to flow through the first and second pipes, respectively, both ata flow rate of 3 l/min.

Subsequently, the hollow fiber membranes were each allowed to passthrough a 50-cm long tank filled with a 50:50 (by parts) mixed solventof water and ethanol and then through a 1.5-m long tank from which warmwater of 60° C. was overflowed, whereby the hollow fiber membranes werewashed. They were dried in a hot air atmosphere, thereby obtaininghydrophilized porous membranes of this invention.

The performance of the porous membranes was evaluated. The results arealso shown in Table 2. The crosslinked hydrophilic polymers were heldalmost uniformly over substantially the entire pore walls of thoseporous membranes.

                                      TABLE 1                                     __________________________________________________________________________                          Performance of hydrophilized porous membrane                            Amount of  Water                                              Crosslinkable monomer                                                                         polymer                                                                             Knot penetration                                                                         Water                                                 Amount used                                                                          held  strength                                                                           pressure                                                                            permeability                                                                            Dissolution (wt. %)                Ex. Kind*                                                                              (wt. parts)                                                                          (wt. %)                                                                             (g/fil)                                                                            (Kg/cm.sup.2)                                                                       (l/m.sup.2 · hr ·                                           mmHg)     1 hr                                                                             24 hr                                                                            200 hr                                                                            800                      __________________________________________________________________________                                                         hr                       Ex. 1                                                                             A    5      10.2  395  0.2   1.3       0.008                                                                            0.010                                                                            0.010                                                                             0.010                    Ex. 2                                                                             A    15     11.0  396  0.2   1.0       0.008                                                                            0.010                                                                            0.010                                                                             0.010                    Ex. 3                                                                             A    80     15.0  395  0.2   0.91      0.007                                                                            0.009                                                                            0.010                                                                             0.010                    Ex. 4                                                                             A    0.5    9.7   390  0.2   0.90      0.045                                                                            0.073                                                                            0.076                                                                             0.076                    Ex. 5                                                                             B    5      4.2   --   0.5   0.55      0.030                                                                            0.032                                                                            0.032                                                                             --                       Ex. 6                                                                             B    15     4.5   --   0.5   0.49      0.058                                                                            0.081                                                                            0.081                                                                             --                       Ex. 7                                                                             B    80     5.7   --   0.7   0.45      0.047                                                                            0.056                                                                            0.066                                                                             --                       Ex. 8                                                                             B    0.5    4.1   --   0.5   0.32      0.26                                                                             0.40                                                                             0.44                                                                              --                       Ex. 9                                                                             A    5      25.1  --   0.2   3.5       0.035                                                                            0.045                                                                            0.045                                                                             --                        Ex. 10                                                                           A    15     27.4  --   0.3   3.5       0.032                                                                            0.042                                                                            0.042                                                                             --                        Ex. 11                                                                           A    80     27.7  --   0.5   3.4       0.028                                                                            0.028                                                                            0.032                                                                             --                        Ex. 12                                                                           A    0.5    23.9  --   0.2   3.0       0.062                                                                            0.088                                                                            0.104                                                                             --                       __________________________________________________________________________     *A: N--hydroxymethylacrylamide                                                B: N,N'--methylenebisacrylamide                                          

                                      TABLE 2                                     __________________________________________________________________________                          Performance of hydrophilized porous membrane                            Amount of  Water                                              Crosslinkable monomer                                                                         polymer                                                                             Knot penetration                                                                         Water                                                 Amount used                                                                          held  strength                                                                           pressure                                                                            permeability                                                                            Dissolution (wt. %)                Ex. Kind*                                                                              (wt. parts)                                                                          (wt. %)                                                                             (g/fil)                                                                            (Kg/cm.sup.2)                                                                       (l/m.sup.2 · hr ·                                           mmHg)     1 hr                                                                             24 hr                                                                            200 hr                                                                            800                      __________________________________________________________________________                                                         hr                       Ex. 13                                                                            C    5      10.3  --   0.5   1.0       0.009                                                                            0.011                                                                            0.011                                                                             0.011                    Ex. 14                                                                            C    15     4.3   --   0.5   0.50      0.052                                                                            0.059                                                                            0.060                                                                             --                       Ex. 15                                                                            D    1      7.5   --   0.5   0.48      0.043                                                                            0.049                                                                            0.050                                                                             --                       Ex. 16                                                                            C    5      8.3   --   0.1   3.5       0.052                                                                            0.058                                                                            0.059                                                                             --                       Ex. 17                                                                            A    5      4.7   --   0.5   0.19      0.016                                                                            0.018                                                                            0.018                                                                             --                       Ex. 18                                                                            A    5      23.0  398  0.8   1.1       0.018                                                                            0.020                                                                            0.020                                                                             --                       Ex. 19                                                                            A    15     23.8  395  0.7   1.1       0.020                                                                            0.022                                                                            0.022                                                                             --                       Ex. 20                                                                            A    1      22.4  392  0.6   0.8       0.065                                                                            0.081                                                                            0.083                                                                             --                       Ex. 21                                                                            A    0.5    22.1  392  0.6   0.6       0.10                                                                             0.13                                                                             0.13                                                                              --                       __________________________________________________________________________     *A: N--hydroxymethylacrylamide                                                C: Triallyl isocyanurate                                                      D: Divinylbenzene                                                        

What is claimed is:
 1. A process for the production of a hydrophilizedporous membrane, which comprises the steps of (A) holding at leastdiacetone acrylamide, a crosslinkable monomer, and a polymerizationinitiator on at least a part of the pore walls of a starting porousmembrane of a polyolefin which has been rendered porous by a stretchingtechnique, and (B) heating them to polymerize these monomers.
 2. Theprocess as claimed in claim 1, wherein the monomers are held on at leastsaid part of the pore walls of the starting porous membrane by preparinga solution of the monomers dissolved in a solvent composed of waterand/or an organic solvent, impregnating the starting porous membranewith the solution and then evaporating the solvent.
 3. The process asclaimed in claim 1, wherein the starting porous membrane is in the formof hollow fibers.
 4. The process as claimed in claim 1, wherein thepolyolefin is a polymer containing as a principal component thereof atleast one monomer selected from the group consisting of ethylene,propylene, 4-methyl-1-pentene and 3-methyl-1-butene.
 5. The process asclaimed in claim 1, wherein the solubility of the crosslinkable monomerin water of 30° C. is 1 g/dl or greater.